(CNN) -- United Airlines and US Airways have joined American Airlines in charging passengers to check bags on certain flights.

Passengers check in at the United Airlines counter at the Miami International Airport Thursday in Miami, Florida.

Citing higher fuel prices, United Airlines said Thursday it will begin charging domestic passengers $15 each way for one checked bag.

The Chicago, Illinois-based airline said the fee to check a second bag will be $25 each way. The fee to check three or more bags, overweight bags or "items that require special handling" will increase from $100 to $125 or from $200 to $250, depending on the item, United said in a statement.

The $15 charge will apply to customers who purchase a ticket on or after Friday for travel within the United States, and who buy on or after August 18 a ticket to or from Canada, Puerto Rico and the U.S. Virgin Islands.

For itineraries that include international flights (except Canada), checking a first and second bag will continue to be free, United said. The cost to check more than two bags or items that are overweight or require special handling varies by destination.

US Airways, meanwhile, said it will begin charging passengers $15 for their first checked bag for travel on or after July 9. The airline said it also has recently started charging passengers $25 for second checked bags. The new policy applies to flights in the United States as well as flights to and from Canada, Latin America and the Caribbean.

Some airlines have been adding fees to once-free benefits, such as snacks. Despite this cost-cutting, airlines are finding it harder to survive as they get squeezed by soaring fuel costs.

"With record-breaking fuel prices, we must pursue new revenue opportunities while continuing to offer competitive fares, by tailoring our products and services around what our customers value most and are willing to pay for," said John Tague, United's executive vice president and chief operating officer.

United said it estimates that the $15 fee will apply to one out of three customers, and the potential revenue from baggage handling service fees, including those for checking a first and second bag, will be approximately $275 million a year.

The move comes a month after American Airlines announced a $15 service charge for the first checked bag.

Delta, Continental and Northwest told CNN they are not charging for passengers' first checked bags. However, a spokesman for Northwest said the airline is well aware of changes in the industry.

"We are always keeping an eye on what our competitors are doing," the spokesman said.